1. Field of the Invention
The invention relates to tin coatings that resist tarnishing and discoloration, and more particularly to a coating made from an antitarnish layer and a tin-base layer deposited over the antitarnish layer. The layers may be combined to form a gradient of antitarnish agent within the tin-base layer. The invention also relates to methods of making the above coatings.
2. Background of the Invention
Oxidation and tarnishing of electrical contact surfaces generally results in high electrical contact resistance, thereby reducing performance of electrical devices. In addition, oxidation and tarnishing of electrical contact surfaces reduces the wettability of solder, and generally makes soldering problematic. Therefore, compositions and methods to reduce oxidation and tarnishing of electrical contacts have been pursued in the art.
One method to reduce oxidation or tarnishing of electrical contacts, and particularly copper or copper alloy leadframes and electrical connectors, is to apply a layer of tin metal onto the copper surface. The tin metal layer acts as a barrier to prevent or reduce oxidation, and thereby maintain the electrical performance of the copper contact. However, under certain conditions (e.g., elevated temperatures in air or other oxygen containing atmospheres), tin coatings have a tendency to oxidize and produce a yellowish oxide film that discolors the surface of the tin coating. Although the oxide film is typically only about 50-200 Angstroms in thickness, the surface of the tin may turn a distinct yellow color, which is undesirable to many consumers. In addition, under certain conditions (e.g., elevated temperature environments), the oxidation layer may attain a thickness that degrades the contact resistance of a coated electrical terminal. The latter conditions are especially prevalent in automobile engine compartments where electrical components such as connectors must endure high local temperatures.
Attempts have been made to address the problem of oxidation of tin coatings. The following patents and publications represent the general state of the art:
U.S. Pat. No. 5,780,172 is drawn to a copper or copper alloy substrate, tin or tin alloy coating layer with a barrier layer between the substrate and the coating layer. The barrier layer is either a copper/nickel layer containing 25%-40% of nickel or a multilayer structure having a copper layer in direct contact with the coating layer.
U.S. Pat. No. 5,916,695 is drawn to a copper or copper alloy substrate, tin or tin alloy coating layer with a barrier layer between the substrate and the coating layer. The barrier layer contains from 20%-40% of nickel and a flash is disposed between the barrier layer and the substrate.
A publication entitled “An Examination of Oxide Films on Tin and Tin Plate” by S. C. Britton and K. Bright discloses that the addition of small amounts of phosphorous, indium or zinc to tin prevents the formation of color films when the metal is heated. Although this article recognizes the need to prevent oxidation of tin, it does not disclose an efficient method to introduce oxide resisting elements into the tin coating such that these elements have an increased concentration at the surface of the tin that is exposed to ambient air.
Japanese Kokai No. 3(1991)-239,353 published Oct. 24, 1991, discloses a copper leadframe for semiconductor devices. This reference describes placing zinc between a copper leadframe and a tin coating. The zinc layer is introduced to prevent diffusion between the tin coating and the copper leadframe. This reference also fails to disclose an efficient method for applying selected oxide resistant elements into a surface of a tin layer exposed to ambient air.
Published PCT Application No. WO 99/51363 discloses a tin coated electrical or electronic component that possesses enhanced resistance to oxidation and tarnishing as well a smaller increase in contact resistance when exposed to elevated temperatures. These benefits are achieved by depositing a relatively thin (e.g., 5-50 angstroms thick) layer of zinc onto the tin coating prior to heating. A subsequent step of heating the sample to a temperature and time effective to convert all free tin to an intermetallic imparts the additional advantage of reducing the coefficient of friction. However, there is a need for improving the robustness and durability of the coating during processing or handling.
As can be seen from the illustrative background discussed above, there is a need for alternative methods of preventing oxidation of tin in electrical conductors and contacts. The present invention is believed to be an answer to that need.